1. Technical Field
The present invention relates to imaging units in electrostatographic/xerographic printing apparatuses. More specifically, the present invention relates to imaging units configured to be removable from a predetermined location in such printing apparatuses.
2. Description of Related Art
In electrostatographic/xerographic printing machines such as photocopiers, printers, and fax machines, a document is set onto a document table situated in the upper surface of the machine. Light is shined on the document image and the light reflected back from the document is used to read the document image. The image information thus read is used to form an electrostatic latent image on the surface of a photosensitive drum. Specifically, a main charging device is used to charge the photosensitive drum at a predetermined polarity, and when the drum is illuminated in accordance with the document image, charge in the illuminated portions is stripped, forming an electrostatic latent image.
A developing device, a transfer device, a separating device, and a cleaning device are disposed around the photosensitive drum.
The developing device contains toner that is charged with the same polarity as the photosensitive drum. In a neighboring position to the developing device and the photosensitive drum, the toner housed in the developing device adheres to the portions of the photosensitive drum that are not charged. Thus, the electrostatic latent image formed on the peripheral surface of the photosensitive drum is pictured into a toner image.
The transfer device applies a potential to the sheet being transported between the photosensitive drum and the transfer device. The potential, whose polarity is opposite that of the toner, is applied through the back of the sheet. This causes the toner on the peripheral surface of the photosensitive drum to be drawn to the sheet, thus transferring the toner image.
A separating device is disposed downstream the sheet transport path from the transfer device. The separating device attracts the back of the sheet away so that it does not wrap around the photosensitive drum. The separating device can be, for example, a separation needle to which a predetermined potential is applied.
In some electrostatographic/xerographic printing machines, the photosensitive drum and the developing device are housed unitarily inside a casing to form an imaging unit. The imaging unit is configured to be withdrawable with respect to the printing machine. Thus the unit can be pulled during maintenance, making the maintenance work simpler. Forming the photosensitive drum and the developing device unitarily is to ensure that the positional accuracy of the components relative to each other is not spoiled, in order not to invite degradation in image quality.
In imaging units such as those described above, the casing is often shaped to match the photosensitive drum, and the developer rollers, stirring paddles and spirals built into the developing device. Thus, the lower surface of the casing may not be formed as a horizontal plane. Wherein such an imaging unit has been pulled out from the printing machine, setting it onto a horizontal surface in the state in which it was housed within the printing machine is difficult. With the imaging unit housed in the printing machine, the toner and developing agent are set such that they remain in their steady state. Not being able to maintain the imaging unit in this state when it is taken out from the printing machine risks that the interior toner and developing agent will not be in their steady state.
Furthermore, the transfer device, which transfers the toner image formed on the peripheral surface of the photosensitive drum to the sheet, is disposed in the vicinity of or in contact with the peripheral surface of the photosensitive drum. Thus, the casing of the imaging unit must be open at least in the position where the transfer device is in the vicinity of or in contact with the photosensitive drum. In electrostatographic/xerographic printing machines in which sideways sheet transport paths are established, the transfer device is often disposed underneath the photosensitive drum. In such cases, the underside of the imaging unit casing is open. Wherein such an imaging unit is taken out of the printing machine, in setting the imaging unit on a horizontal surface, maintaining the state the imaging unit is in when housed in the printing machine is difficult. In particular, although the photosensitive drum is exposed on the underside of a casing thus configured, it should be kept from contacting anything while out of the machine.